1. Field of the Invention
The present invention relates to an exposure apparatus to manufacture a device such as a semiconductor device or liquid crystal device.
2. Description of the Related Art
The process of manufacturing a semiconductor device formed of a micropattern such as LSIs or VLSIs employs a reduction projection exposure apparatus which reduces and exposes a circuit pattern formed on a mask (reticle) onto a substrate (wafer) coated with a photosensitive agent to form an exposed pattern.
As the integration density of the semiconductor device increases, further pattern miniaturization is required. Demands for a development in resist process as well as for miniaturization for the exposure apparatus are increasing simultaneously.
A method of improving the resolution of the exposure apparatus includes a method of changing an exposure wavelength to a shorter wavelength and a method of increasing the numerical aperture (NA) of a projection optical system. When the resolution is improved in this manner, the depth of focus of the projection optical system decreases. Hence, an increase in focus accuracy with which the wafer surface is matched with the imaging plane (focal plane) of the projection optical system is an important issue.
One of the important optical characteristics of the projection exposure apparatus is alignment accuracy with which the patterns in a plurality of steps are overlaid accurately. An important factor that influences the alignment accuracy includes a magnification error of the projection optical system. The pattern feature size of a VLSI shrinks every year, and accordingly a requirement for an improvement in alignment accuracy also increases. Hence, it is very important to maintain the magnification of the projection optical system at a predetermined value.
It is known that the projection optical system absorbs part of the exposure energy, and that a heat generated by the absorption changes the temperature of the projection optical system, which, in turn, changes the optical characteristics such as the refractive index of the projection optical system.
When the projection optical system is irradiated with exposure light over a long period of time, the imaging characteristics (focus, magnification, distortion, astigmatism aberration, wavefront aberration, and the like) of the projection optical system fluctuate. Consequently, non-negligible errors of focus and alignment as described above may undesirably occur.
In view of this, a method has been proposed which compensates for the fluctuation in imaging characteristics which occurs in the projection optical system depending upon the exposure energy irradiation state. For example, according to Japanese Patent Publication No. 63-16725 of the present applicant, the fluctuation amount of the imaging characteristics depending on the exposure energy state of the projection optical system is calculated by a model expression including the exposure amount, exposure time, non-exposure time, and the like as parameters, and the fluctuation in imaging characteristics of the projection optical system is corrected based on the calculation result.
The model expression described above has coefficients for the respective imaging characteristics specific to the projection optical system. The fluctuation of the imaging characteristics of the projection optical system can be obtained and corrected by setting the coefficients approximately.
An exposure apparatus has been proposed which can obtain a higher resolution for a specific projected pattern by changing the illumination shape. In this apparatus, the light source distribution formed on the pupil plane of the projection optical system changes depending on the exposure condition (the NA of the projection system, the numerical aperture of the illumination system, the exposure region, the exposure central position, the mask used for exposure, and the like). Accordingly, the fluctuation amount of the imaging characteristics for each exposure condition changes.
Hence, an exposure method has been proposed which adjusts the imaging characteristics fluctuation well even when the distribution of an energy incident on the projection optical system changes. For example, according to a method disclosed by Japanese Patent No. 2828226, the correction coefficient of the imaging characteristics corresponding to the light source distribution state of the illumination light is stored. When the light source distribution state is changed, corresponding correction information is read out, and the fluctuation is corrected based on the readout information.
To accurately correct the fluctuation of the imaging characteristics corresponding to the light source distribution state of the illumination light described above, a correction coefficient optimum for a given exposure condition must be calculated from a difference in light source distribution state of the illumination light on the pupil plane, reticle transmittance, exposure region, scanning speed, exposure amount, irradiation time, and the like.
As described above, a correction coefficient which is optimum for the exposure condition and serves to compensate for the fluctuation of the imaging characteristics must be calculated. As the miniaturization progresses, an improvement in accuracy is required of the exposure apparatus. To correct more accurately the fluctuation of the imaging characteristics occurring upon exposure is sought for.
The correction coefficient changes for each exposure condition, and it is difficult to completely suppress thermal fluctuation accompanying exposure. It requires a constant length of time or more to capture a thermal fluctuation phenomenon, and it requires much time to obtain correction coefficients for a plurality of exposure conditions.
Assume that correction coefficients for the respective exposure conditions are to be calculated separately. To eliminate the influence of exposure under each exposure condition, evaluation under the next exposure condition must be performed after the fluctuation of the imaging characteristics caused by exposure under a previous exposure condition almost disappears. This requires much time due to the characteristics of the thermal relaxation phenomenon.